This invention relates to silicone rubber compositions and methods for the preparation thereof. More particularly, this invention relates to a highly flame-retardant silicone rubber composition that also has good electrical insulating characteristics and to a method for the preparation thereof.
Silicone rubbers find frequent use in a variety of applications as a consequence of their excellent heat resistance, excellent resistance to aging, excellent electrical properties, and the like. However, a drawback to silicone rubbers is their flammability, and in response to this numerous methods have been proposed for making silicone rubbers self-extinguishing (flame retardant). For example, Japanese Patent Publication (Kokoku) Number Sho 44-2591 (2,591/1969) teaches the composition afforded by blending a platinum compound, such as chloroplatinic acid or a platinum-olefin complex, into a silicone rubber compound. However, these platinum compounds alone do not provide an acceptable flame retardancy, and this fact has resulted in a good deal of research into the combination of these platinum compounds with inorganic filler in order to improve the flame retardancy. As examples of this, Japanese Patent Publication (Kokoku) Number Sho 47-21826 (21,826/1972) teaches a composition in which a platinum compound and fumed titanium dioxide are blended into a silicone rubber compound; Japanese Patent Publication (Kokoku) Number Sho 51-23979 (23,979/1976) teaches a composition in which a platinum compound and manganese carbonate micropowder are blended into a silicone rubber compound; and Japanese Patent Publication (Kokoku) Number Sho 51-35501 (35,501/1976) discloses a composition in which a platinum compound and iron oxide are blended into a silicone rubber compound.
Nevertheless, even these silicone rubber compositions do not exhibit a fully satisfactory flame retardancy. Moreover, they each suffer from other, specific problems. The composition containing a platinum compound and fumed titanium dioxide yields a silicone rubber whose electrical properties deteriorate in a humid environment. The composition containing a platinum compound and manganese carbonate suffers from an inhibited cure, and hence a failure to cure thoroughly, when an acyl-type organoperoxide is used as its curing agent. Finally, the composition containing a platinum compound and iron oxide does not exhibit an improved flame retardancy in the absence of large additions of iron oxide, but at the same time these large additions cause a diminished mechanical strength in the resulting silicone rubber.
Other tactics known for achieving improvements in the flame retardancy of the subject silicone rubber compositions consist of increasing the addition of the nonflammable inorganic filler and reducing the addition of the flammable polyorganosiloxane gum. Unfortunately, the silicone rubber compositions afforded by these tactics have a poor molding processability, and their thermal cure affords silicone rubber moldings with a reduced mechanical strength. These drawbacks place limitations on the applications of these compositions.
As a result of extensive investigations directed to solving the problems described above, the inventors discovered that the blending of special additive components into a specific silicone rubber composition affords a cured silicone rubber with a vastly improved flame retardancy and also improved electrical insulation characteristics as typified by the tracking resistance. The present invention was achieved based on this discovery.
In specific terms, an object of the present invention is to provide a silicone rubber composition that cures into a highly flame-retardant silicone rubber with excellent electrical insulation characteristics and that does so without sacrificing mechanical strength. An additional object of the present invention is to provide a method for the preparation of said silicone rubber composition.